Molded articles of conductive resin compositions have recently been anticipated as materials to replace conventional metalwork products. In the electrical field, molded articles of conductive resin compositions have become of major interest as materials which are excellent in moldability and corrosion resistance and are also cheap. Examples of the material in the electrical field include resin compositions having high conductivity used as separator materials for a fuel cells, and various cell members.
The fuel cells are classified depending on the kinds of the electrolyte, the fuel and the oxidizing agent which are used ins portions of constituent members. The fuel cell includes, for example, a solid polymer type fuel cell comprising a solid polymer electrolyte membrane as an electrolyte, a hydrogen gas as a fuel, and air as an oxidizing agent; and a methanol direct type fuel cell wherein hydrogen is directly derived from methanol in the fuel cell and is used as the fuel. These fuel cells can perform efficient power generation at comparatively low operation temperature of 200° C. or lower on power generation.
A separator as one of constituent members of these fuel cells requires gas impermeability for stable feeding of a fuel and an oxidizing agent gas to electrodes in a separated state, and conductivity for enhancement of power generation efficiency, and also requires durability such as corrosion resistance under the operation environment, or hydrolysis resistance. In the production of the separator, excellent productivity and good moldability are required.
As materials for these separators for a fuel cell, for example, the use of molding materials made of carbon-based fillers and thermosetting resins, and molded articles has been studied.
It has been proposed to use, as the thermosetting resin, epoxy resin, phenol resin, and radical polymerizable resin.
Among these resins, a radical polymerizable resin is excellent in view of productivity on molding and various techniques using such a resin have been proposed. Japanese Unexamined Patent Application, First Publication No. 2001-151833 proposes, for example, a curable composition comprising a vinyl ester resin and a carbon-based filler. Also, Japanese Unexamined Patent Application, First Publication No. 2002-164063 proposes a resin composition comprising a radical polymerizable resin such as vinyl ester resin, and a carbon-based filler.
According to these techniques, the resulting separator is excellent in initial performances such as mechanical strength and conductivity, but is inferior in corrosion resistance, particularly durability such as hydrolysis resistance. Since an epoxy (meth)acrylate as a vinyl ester resin which is used in the technique disclosed in the document generally has a lot of hydroxyl groups in the molecule, structurally, the molded article made of the resin has high water absorption and the strength decreases due to water absorption. Furthermore, hydrolysis of the resin proceeds by water absorbed and the strength of the molded article decreases, and thus such an epoxy (meth)acrylate is not suited for practical use in a separator for a fuel cell.
Japanese Unexamined Patent Application, First Publication No. 2002-164063 discloses, as a method for thickening the vinyl ester resin on molding, a method of adding an polycarboxylic acid anhydride to a hydroxy group formed by reacting an epoxy group with a carboxyl group to form a carboxyl group and thickening using an oxide of metal such as magnesium or calcium as a thickener. However, this method causes problems such as deterioration of water resistance due to elution of metal and adverse influence on fuel cell properties, and thus the resulting fuel cell is not suited for practical use.
U.S. Pat. No. 6,251,308 proposes a curable resin composition comprising a vinyl ester resin, a carbon-based filler, and a polyisocyanate. The resulting separator made of this resin composition is excellent in mechanical strength and conductivity, and durability such as hydrolysis resistance is improved to some extent; however, water absorption of the resin and decrease in mechanical strength of the molded article due to hydrolysis may occur according to the composition of the resin such as unsaturated polyester, and thus there is yet room for improvement in durability, particularly hydrolysis resistance.
It is not necessarily easy to control moldability of the resin composition and in the case in which a molded article having a complicated shape such as a separator for a fuel cell is obtained by a compression molding method as a molding method disclosed in the document, the resin composition as the molding material may not be fully filled into the portion having a complicated shape of the mold, or warp may occur in the resulting molded article, and thus the resulting molded article does not have stable quality in view of moldability.
As described above, there has not yet been made a conductive resin composition which is excellent in filling into a mold having a complicated shape, and also ensures high-level mechanical strength, conductivity, gas impermeability, corrosion resistance, and durability such as hydrolysis resistance of the resulting molded article.
An object of the present invention is to provide a separator for a fuel cell which is excellent in dimensional accuracy, conductivity, heat resistance, and mechanical strength, and is also excellent in durability such as hydrolysis resistance.
Another object of the present invention is to provide a conductive resin composition which does not cause problems with regard to moldability such as occurrence of separation between a resin component and a conductive filler, voids and warp on molding, and is excellent in filling into a mold, and is also capable of being used to produce various electrical and electronic materials, including a separator for a fuel cell having excellent properties.
Another object of the present invention is to provide a method for producing the conductive resin composition.